This invention relates to a method and apparatus for grinding camshafts.
Specialized grinding machines and methods have previously been used to grind the lobes on engine camshafts. The use of these specialized machines is necessary due to the irregular configuration and angular orientation of the cam lobes. The irregular cam lobe configuration makes it necessary to move the camshaft toward and away from a grinding wheel as the camshaft is being rotated with the grinding wheel in engagement with a lobe on the camshaft. In order to provide for this movement of the camshaft, the headstock and tailstock which rotate the camshaft have been mounted on a rocker bar which is pivoted toward and away from the grinding wheel by a cam follower and master cam assembly. Biasing springs have been utilized to urge the rocker bar toward the grinding wheel and to maintain the cam follower and master cam assembly in engagement during a grinding operation.
Since the movement of the rocker bar toward and away from the grinding wheel will vary with the configuration of the particular cam lobe which is being ground, the master cam assembly commonly includes a plurality of cam elements with at least one cam element for each lobe on the camshaft. Upon completion of the grinding of a cam lobe, the work table or carriage is moved relative to the grinding wheel to move the next succeeding cam lobe into alignment with the grinding wheel. This indexing operation has been performed with the rocker bar in a fully retracted or loading position in which the cam lobes are spaced from the grinding wheel and in which the master cam assembly is spaced as far as possible from the cam follower. As the next succeeding cam lobe is moved into alignment with the grinding wheel, a dog on the base of the grinding machine actuates a star wheel to move the cam follower into alignment with the master cam which is associated with the next lobe on the camshaft. Typical of these known grinding machines are the grinding machines disclosed in U.S. Pat. Nos. 2,535,130 and 2,786,311.
These known grinding machines have been very satisfactory in their general mode of operation. However, it is desirable to increase the accuracy, ease of manufacture and use, and the operating speed of these known camshaft grinding machines.
The accuracy with which a known camshaft grinding machine is effective to grind the lobes on a camshaft is, in part, determined by the accuracy with which the master cam assembly and cam follower move the rocker bar toward and away from the grinding wheel. During a rough grinding operation, marterial is removed at a relatively high rate from the lobe on the camshaft. This high rate of material removal results in relatively large operating forces being present between the grinding wheel and camshaft. In order to overcome these relatively large operating forces, large biasing forces have been utilized to urge the rocker bar toward the grinding wheel. These relatively large rocker bar biasing forces also press the master cam assembly and cam follower into abutting engagement.
Due to the relatively large magnitude of the rocker bar biasing forces required for a rough grinding operation, the forces pressing the master cam assembly against the cam follower can result in deflection of components of the grinding machine in such a manner as to introduce inaccuracies in the grinding of the cam lobes. These inaccuracies are relatively small and are not excessively troublesome during rough grinding of a cam lobe. However, during finish grinding of a cam lobe, even the slight inaccuracies introduced by the large rocker bar biasing forces are objectionable.
With known camshaft grinding machines in which the cam follower is indexed relative to the master cams by engagement of a star wheel with dogs mounted on the base of the grinding machine, the cam follower must engage the master cam elements in a sequence which is the same as the sequence of the corresponding lobes on the camshaft. In addition, the dogs must be accurately adjusted to provide the desired indexing movement of the cam follower relative to the master cam assembly. It is possible for the known star wheel drive arrangement to malfunction so that the cam follower is not indexed by one of the dogs. Of course, this results in the cam follower being misaligned relative to the master cam assembly so that the cam lobes are not ground to the desired configuration.
During use of these known grinding machines, the rocker bar is moved from a loading position to a range of operating positions by a motor which is connected with the rocker bar. Each time the grinding of a lobe on a camshaft has been completed, the rocker bar is pivoted through a relatively large distance from the range of operating positions back to the loading position. Before the next cam is ground, the rocker bar must be pivoted back to an operating position. If the rocker bar motor is operated at a relatively high speed to pivot the rocker bar from the loading position to an operating position, objectionable impact forces may be present between the master cam assembly and cam follower.